LABYRINTH 567 



the posterior surface of the petrous part of the temporal bone, 

 where it opens under the dura mater. 



Canales Semicirculares Ossei. These are three bony canals 

 or tubes placed posterior to the vestibule. They are bent 

 upon themselves, so that each forms considerably more than 

 half a circle, and they occupy planes at right angles to each 

 other like three faces of a cube. They are termed superior, 

 posterior, and lateral, and they open into the posterior part 

 of the vestibule by five round orifices, the number of openings 

 being thus reduced through the adjoining extremities of the 

 superior and posterior canals becoming fused together so as 

 to present a common canal, the crus commune, with a single 



Superior semicircular canal 

 with its ampulla 



Cochlea 



Fenestra cochlea; 



Fenestra vestibuu '- I Posterior semi- 



Ampulla of posterior semicircular canai | circular canal 



Ampulla of lateral semicircular canal 1 Crus commune 



Lateral semicircular canal 



FIG. 249. Left Bony Labyrinth viewed from lateral side. (Howden. ) 



orifice. One extremity of each canal where it joins the 

 vestibule becomes expanded into what is termed its ampulla. 

 There are thus three ampullated ends. 



The superior semicircular canal forms the highest part of 

 the labyrinth, and gives rise to a smooth elevation on the 

 anterior surface of the petrous part of the temporal bone, 

 immediately anterior to its superior angle. It is vertical, 

 and placed almost transversely to the long axis of the petrous 

 part of the temporal bone. The posterior semicircular canal, 

 which is the longest of the three tubes, is also vertical, and 

 lies in a plane parallel to the posterior surface of the petrous 

 part of the temporal bone. The lateral semicircular canal is 

 the shortest of the tubes, and it lies in a horizontal plane. 



Cochlea. The cochlea is a tapering tube which is coiled 



